Belt sanders are fast-working, powerful machines that remove stock and old finishes much faster than do orbital-type sanders. Most homeowners and do-it-yourselfers have preferred, orbital sanders because they are lighter, less expensive and do not require such careful handling when working on furniture or cabinets. Because belt sanders cut so fast, there is always the danger of taking off too much material if they are not carefully handled.

Now there is a new lightweight, compact belt sander that eliminates most of the objections the do-it-yourselfer may have to a belt sander. Called the Sandcat and made by the Skil Corporation, 4801 W. Peterson Ave., Chicago, Ill. 60646, it uses smaller sanding belts -- 2 1/2 inches by 16 inches -- and weights only 4 1/2 pounds. The light weight and compact design make the tool easy to use with one hand, even when working overhead, and there is a removable knob on the front for two-handed operation.

The Sandcat, which costs $54.99, is double insulated for protection against shock. The belt moves at a slower speed than conventional belt sanders so there is less chance of gouging or excess sanding, yet it has enough power and speed to permit much more rapid stock or paint removal than is possible with an orbital sander. An accessory dust collector attachment can be connected to a tank-type vacuum cleaner to control sanding dust. Air Pump

Ideal for pumping air into automobile and bicycle tires, all kinds of inflatable toys, pools, footballs and basketballs, the new Miracle Air Pump is compact and all metal and can be powered by any 1/4-inch or 3/8-inch portable electric drill. The air hose comes with a quick-connect, locking-type valve that snaps onto bicycle or automobile tire valves, and accessory inflator attachments are furnished for use with sports equipment, toys, inflatable boats and other equipment without a conventional tire valve.

The pump is made by Miracle Products, Inc., Plymouth Industrial Park, Terryville, Conn. 06786, and costs $24.95. It will build pressures up to 90 pounds per square inch, and is capable of pumping up to 1 cubic foot of air per minute, according to the manufacturer. Water Alarm

To prevent much of the costly damage caused each year by flooding of basements, laundry rooms, bathrooms and other places where a leak may cause extensive water damage if not promptly detected and stopped, and to warn the homeowner if a sump pump fails to start automatically when it should (due to a stuck float switch, a power failure or a corroded control), two new battery-powered, water-sensing alarm units can be of great help to all homeowners.

The first is Flood Alert, made by Datasonic, Inc., 255 East Second St., Mineola, N.Y. 11501. This compact unit is self-contained and requires no installation. It is placed face up on the floor or other surface where it is needed to detect water -- for example, on a basement floor, next to a boiler or furnace that could flood, next to a washing machine, under a bathroom toilet or near a sump pump to warn if it overflows.

Two metal contacts sense moisture and sound a loud, high-pitched beep to give immediate warning of water accumulating. Costing $16.95 postpaid, the Flood Alert is powered by a 9-volt battery that will sound an alarm for up to five days if the battery is fresh. It is a little more than five inches long and can be equipped with optional remote sensors ($4.95) that enable it to monitor several widely separated locations with one unit.

The second new unit, Water Alarm, is made by Northern Electric Co., 5224 North Kedzie Ave., Chicago, Ill. 60625. It costs $29.95 and is similar to the Floor Alert in that it is powered by a 9-volt battery and is placed on the floor or other surface to be protected.

This unit is round instead of rectangular and slightly larger, but its circuitry includes a low-battery indicator that gives intermittent beeps to warn when the battery is getting too low to operate the alarm. Self-Storing Extension Cord

Electric extension cords are necessary around most homes where portable power tools and other electrical appliances are used inside or outside the house. Coiling and uncoiling these long cords, especially heavy-duty types recommended for use with most power tools, has always been a nuisance. A new product, Cord Caddy, solves this problem neatly.

Made by Beck Electric Manufacturing, Inc., 737 Canal St., Stamford, Conn. 06902, the unit contains 25 feet of heavy-duty, three-conductor wire (No. 14). Two three-prong, grounded outlets are next to the rewind handle in the center and the unit is UL-listed. Turning the handle winds the cord on the inside. Pulling on the end enables the user to reel out as much as needed, then plug tools or appliances into one of the two outlets. The Cord Caddy costs $29.95 with built-in 10-amp circuit breaker, and $25.95 without the circuit breaker.